1. Field of the Invention
The invention relates to the high pressure polymerization of ethylene with free radical catalysts.
2. Description of the Prior Art
It is known to polymerize ethylene, alone or with other comonomers, under high pressure conditions in an elongated tubular reactor for the purposes of producing polymers of ethylene having both a relatively narrow density range of about 0.850 to 0.940 and a relatively narrow melt index range of about 0.05 to 10,000. Such properties are needed for film and other grade resins. It is also known that free radical initiators or catalysts, such as oxygen, azo compounds and organic peroxides, may be used to catalyze the polymerization of the monomer charge under such polymerization conditions to produce such polymers. In order to help regulate the molecular weight, or melt index, of the ethylene polymers, it is also common to employ chain transfer agents in the high pressure polymerization reaction system. The percent conversions which are obtained with such catalysts under such conditions, however, are usually relatively low, i.e., of the order of only about 10 to 20 percent. ##EQU1##
In order to attempt to improve the percent conversion of monomer to polymer during the passage of the monomer through the reaction tube it has been known to use a plurality of free radical initiators. Although the percent conversion of monomer to polymer may be thereby improved, it has been found, however, that because of the use of the plurality of initiators, in combination with the chain transfer agents, in such reactors, it has not been readily possible to provide for an increased percent conversion of monomer to film grade polymer without also suffering a concomitant loss in the optical properties of film made from the resin having specified melt index and density properties.
In addition to losses in optical properties, such as gloss and clarity, polymer made in some cases with a plurality of initiators also tends to have an undesirable, high hexane extractable fraction i.e. a low molecular weight fraction.